String to number

 // File: convert.h
 #include <iostream>
 #include <sstream>
 #include <string>
 #include <stdexcept>
 
 class BadConversion : public std::runtime_error {
 public:
   BadConversion(const std::string& s)
     : std::runtime_error(s)
     { }
 };
 
 inline double convertToDouble(const std::string& s)
 {
   std::istringstream i(s);
   double x;
   if (!(i >> x))
     throw BadConversion("convertToDouble(\"" + s + "\")");
   return x;
 }